How do health insurance companies determine what doctors are in-network? why are some doctors affiliated with in-network hospitals but still not considered in-network physicians?

Different groups: Doctors with permission to use a particular hospital to care for patients are said to "have privileges" at that hospital. Hospitals need doctors, and some doctors need hospitals, but affiliations usually do not involve finances. Doctors and hospitals bill you separately. Insurance networks are mainly a money issue. Insurers pay different amounts if a doctor joins them vs. "staying independent."

In network: means the doc or the practice has been accepted by an insurance carrier and signed a contract to accept specific fees for specific procedures. Sadly, much of healthcare is simply big business. Peace and good health.